A secondary liquid-electrolyte battery generally contains at least one electrochemical battery cell that include a negative electrode, a positive electrode, and a separator situated between the electrodes. The negative and positive electrodes are constructed from materials that can participate in both oxidation and reduction reactions. Such materials allow an electric current to be reversibly passed between the electrodes, external to the separator, while an ionic species migrates through the separator within a liquid electrolyte to electrochemically balance the current flow. This type of electrical and ionic current flow occurs spontaneously (cell discharge phase in which oxidation occurs at the negative electrode and reduction occurs at the positive electrode) or is compelled (cell charge phase in which oxidation occurs at the positive electrode and reduction occurs at the negative electrode). The electric current generated during cell discharge may be used to power, at least in part, an electrical load while an applied voltage from an external power source may be used to charge, or re-power, the cell once its current capacity has fallen to an undesirable level.
The separator facilitates operation of the electrochemical battery cell by providing a porous and electrically-insulative mechanical support barrier between the two electrodes. The separator, in general, has a porosity sufficient to contain the liquid electrolyte—which can communicate the ionic species—yet is thermally, chemically, and mechanically stable enough to separate the negative and positive electrodes over the course of many discharge/charge cell cycles so that a short-circuit is prevented. A wide variety of materials, either alone or in combination with one another, have been either utilized or investigated for construction of the separator with the goal of imparting long term operational reliability to the separator within different working environments. The most commonly used separators today are made from a single flat polyolefin sheet membrane or a laminate of several flat polyolefin sheet membranes. The particular polyolefins usually employed are those derived from simple low-carbon number olefins, such as polypropylene and polyethylene.
The electrochemical battery cell, in order to interact with the electrical load and the external power source, is configured for connection to an external circuit that provides an electric current path between the negative and positive electrodes around the separator. Each of the negative and positive electrodes, for instance, is typically associated with a metallic current collector that helps distribute the electric current passing through the external circuit to and from all electrochemically active regions of the electrodes. A connection feature such as connector tab may be included on each of the metallic current collectors. The connection feature may protrude away from the electrochemical battery cell to operatively establish an electrical connection with the external circuit. This is usually accomplished by connecting the protruding connection features associated with the negative and positive electrodes to negative and positive terminals, respectively, in either a serial or parallel relationship with the connection features associated with other electrochemical battery cells. Negative and positive terminals may not be needed, however, if the secondary liquid-electrolyte battery includes only one electrochemical battery cell.
The connection feature included on a metallic current collector is commonly located near a peripheral edge of the current collector for various practical reasons including, among others, accessibility. But locating the connection feature in this way can cause an uneven current density distribution to develop within the electrochemical battery cell. For instance, during oxidation of the associated electrode, the connection feature may have a tendency to draw electric current from a nearby portion of the electrode at a greater rate than more distant portions. Likewise, during reduction of the associated electrode, the connection feature may have a tendency to make electric current more readily available for ionic species reduction at a nearby portion of the electrode as opposed to other portions further removed. Such differences in electrochemical activity can become even more pronounced if the connection features of the metallic current collectors are all located on the same side of the cell. Nonetheless, however developed, a region of the electrochemical battery cell that experiences disparately greater current density can result in some potentially undesirable effects over the life of the battery.